We analyze the performance of Japanese open-type stock mutual funds for the 1981-1992 period. The results show that, regardless of the performance measures and benchmarks employed, most of the Japanese mutual funds underperform the benchmarks by between 3.6% to 10.8% per annum. These funds tend to invest more in large stocks with low book-to-market ratios. But this feature does not explain the underperformance. A potential explanation is the dilution effect caused by inflows of funds. In Japan, a new investor of an open-type fund only pays in the after-tax value of the net asset value. We conduct a bootstrap experiment to assess the magnitude of this dilution effect.